One of the main difficulties in sheet metal bending is determining the appropriate flat length of a piece so that it has the desired outside surface dimensions after it is bent. This is difficult because the bending process causes the inside surface of the metal to compress, and the outside surface to stretch. The amount of stretching that occurs is difficult to predict. Historically this problem has been addressed through the use of tables developed for various types and thicknesses of material based on test bends done with the material. The current method used to account for this stretching, and thus calculate the appropriate flat length, is called Bend Deduction. The Bend Deduction value is how much the sum of the two desired flange dimensions should be reduced by to get the correct fiat length. The main advantages of this method are that it is cheap and simple—one only has to measure the flange lengths A and B and the flat length of the part using calipers. However, it also has many disadvantages. The most significant problem is that if the Bend Angle is not 90 degrees, the flange lengths A and B cannot be measured accurately. Also, the Bend Deduction value depends on the desired Bend Angle, so even if an accurate Bend Deduction value is calculated for a 90 degree bend, one cannot predict what the Bend Deduction value will be for a different Bend Angle.
Another way to calculate the appropriate flat length for a part is by determining the location of the neutral line. During the bend, the material on the inside surface is compressed and the material on the outside surface is stretched. Thus there must be some location between the inside surface and the outside surface where there is no stretching or compression. This line where the molecules of the material are neither stretched nor compressed is called the neutral line. The neutral line is located at a distance t from the inner surface of the work piece. The inner surface will include the interior angle after bending.
The most important aspect of the neutral line is that its length is equal to the flat length of the piece. Thus, if we know where the neutral line is located we can determine exactly how long our flat piece should be in order to obtain the correct bent dimensions. Another important feature of the neutral line is that its location does not depend on the bend angle. This is due to the fact that once a bend has started, material that is already compressed will not start to stretch, and material that is already stretched will not start to compress, thus increasing the angle of the bend will have no effect on the location of the neutral line.
If the distance t is known, it can be used to calculate the correct flat length for any desired Bend Angle. However, the neutral line is located within the material, so obviously the distance t cannot be measured using a conventional method such as calipers. This difficulty in measurement is the reason that the Bend Deduction method is used instead of the neutral line method. There simply is no easy way to determine the location of the neutral line for a bend on the shop floor. The present invention addresses these limitations.
Various inventions have been developed to address the problem of determining the starting work piece size for a bent metal construction of a specified size.
Lascoe, O. D., Handbook of Fabrication Processes”, ASM International, 1988, pp. 187 and 189 includes a chapter on Bending of Sheet Metal. This chapter includes a section on Bending Calculations. This section states that a common error in determining blank lengths is the failure to add or subtract the sheet-metal thickness when necessary. This section also states as a general rule in blank development is to divide the part into straight sections and bends or arcs. Then the length of each section is found. Often it is necessary to draw in right triangles to connect known to unknown dimensions. Trigonometry is then used to solve for an unknown side or angle. FIG. 2G-11 illustrates many bending terms including a Bevel angle (B).
Leigh, R. W., “Bend Allowance Formulas”, http://ronleigh.com/ivytech/_ref-ba.htm, copyright 1994, 2006; revision Dec. 5, 2008 discloses two formulas:
an empirical formula for a K-factor as:K=((R/T)/16)+0.25
and a Bend Allowance formula:B.A.=A·π·(R+K·T)/180 (A measured in degrees)
Both of these equations are derived in this reference. The K-factor equation is derived from experimental data. The Bend Allowance is an equation for the arc length of the neutral axis through the bend given the bend angle A and the adjusted radius as the inside radius, R, plus the distance to the neutral axis, t, given by t=K·T, where T is the thickness of the material.
Diegel, O., “BendWorks The fine-art of Sheet Metal Bending”, Complete Design Services, July 2002 discloses equations for the Bend Allowance, Bend Deduction and k-factor. Using a test sample, this reference discusses reverse engineering the k-factor by measuring the total flat length, the outside lengths of the bent section, the bend radius, the bend angle and the thickness of the material.
U.S. Patent Patent Application No. 2010/0106463, published for Hindman et al. is directed to custom equations for the unfolding of sheet metal. This system provides the ability to utilize custom equations for the unfolding of sheet metal and to determine how sheet metal bends. The custom equation solution allows users to define unfolding expressions based upon equation types that provide a reference to how the expressions will be geometrically based. The equation type may be selected by the user and can be from among a list of available types including bend allowance, bend compensation, bend deduction, and k-factor. In this regard, the equation type may be selected from the four types and appropriate equations are displayed with variables that may be customized.
U.S. Pat. No. 5,689,435, issued to Umney et al. is directed to systems and methods for automated bracket design. This reference discloses equations for the bend allowance and for the bend deduction.
U.S. Pat. No. 5,842,36, issued to Hans Klingel et al. discloses as part of a process for bending work pieces, when the work piece is released from the upper die and/or the lower die, the actual size of the bending angle is continually determined and from the actual size of the bending angle found, the change in it is determined and, as soon as the change in the actual size of the bending angle assumes a predetermined value, the actual size of the then existing bending angle is compared with the desired size. On a tooling machine for carrying out the method described, there are scanning elements and a device for determining the actual size of the bending angle that are parts of a device for determining the change in the actual size of the bending angle. The device for determining the actual size of the bending angle is connected to a comparison device for comparing the actual size of the bending angle to the desired size.
U.S. Pat. No. 7,643,967, issued to Max W. Durney et al., discloses A method of designing fold lines in sheet material includes the steps defining the desired fold line in a parent plane on a drawing system, and populating the fold line with a fold geometry including a series of cut zones that define a series of connected zones configured and positioned relative to the fold line whereby upon folding the material along the fold line produces edge-to-face engagement of the material on opposite sides of the cut zones. Alternatively, the method may include the steps storing a plurality of cut zone configurations and connected zone configurations having differing dimensions and/or shapes, defining a desired fold line in a parent plane on a drawing system, selecting a preferred cut zone and/or a preferred connected zone which have a desired shape and scale, locating a preferred fold geometry along the fold line, the preferred fold geometry including the selected cut zone and the selected connected zone, and relocating, resealing and/or reshaping the preferred fold geometry to displace, add and/or subtract at least one of the connected zones, whereby upon folding the material along the fold line produces edge-to-fact engagement of the material on opposite sides of the cut zones. A computer program product and a system configured for implementing the method of designing fold lines in sheet material is also disclosed.
It is an objective of the present invention to provide a method for accurately determining the starting size for a piece of sheet metal that is to be bent into a bent construction of a specified size. It is a further objective to provide such a method that can be repeatedly used for sheet metal of various thicknesses and materials. It is a still further objective of the invention to provide a method that can be accurately applied to sheet metal bends of any angle. It is yet a further objective to provide such a method that is easy to use and that requires a minimum of equipment. Finally, it is an objective of the invention to provide a method to accurately determine the location of the neutral line for any thickness of any work piece of any material.
While some of the objectives of the present invention are disclosed in the prior art, none of the inventions found include all of the requirements identified.